This was one of those things that I couldn’t believe the Magic Mouse didn’t handle automatically for you. However, you can install Magic Prefs for free. With in there you can add ActionScript. It isn’t very clear how to do this.
First copy this to your clipboard:
tell application “iTunes”
Then, choose a mouse action, follow menus down to Custom Actions => Manage ActionScript
Just type a name into the top box and click the +
You will then see it in your list of available actions for the mouse.